1. Field of the Invention
The present invention relates to semiconductor electronic devices, and, more particularly, to monolithic semiconductor broadband switches which could be used, for example, in broadband communication and radar transmit/receive modules.
2. Description of the Related Art.
Cost and reliability pressures in microwave communication and radar systems have demanded the integration of various components such as power amplifiers, low noise amplifiers, transmit/receive switches, and phase shifters on single monolithic semiconductor chips. As the number of integrated components increases, the size (chip area occupied by) each component becomes more significant; and there is a need to reduce component size.
Phased array radars include hundreds or thousands of transmit/receive modules in a matrix with about one quarter-wavelength spacing between modules; the radar beam is shaped by combining the outputs of each of the modules and is electronically steered by controlling the phase of each module. FIG. 1 is a schematic layout of a module; each of the transmit/receive switches is a single-pole, double-throw (SPDT) switch. The size, weight, power consumption, manufacturability, and cost of such a radar would be prohibitive without monolithic integration of the modules.
Known broadband SPDT switches typically use p-i-n diodes or MESFETs as shunts in connection with transmission line sections of approximately quarter-wavelength; however, such switches occupy large chip area. See for example, W. Davis, Microwave Semiconductor Circuit Design .sctn. 13.4 (van Nostrand Reinhold 1984) which describes p-i-n diode SPDTs based on the maximally flat quarter-wave coupled bandpass filter. The p-i-n diode-based switch operates by applied dc bias toggling the diodes between a low resistance state and a small series capacitance state. Also, FIGS. 2A-B illustrate in schematic and plan layout views an SPDT for 2-20 GHz with MESFET active elements; for the open pole, the series MESFET is pinched-off and the shunt MESFETs are turned on which provide low impedances at points about a quarter-wavelength (higher frequency) from the input port and thus additional isolation at high frequencies. (The precise locations for the shunt MESFETs are determined by optimization with computer simulation.) The closed pole has the MESFET gate biases reversed and matches to 50 .OMEGA.. However, the switch occupies considerable area due to the transmission line segments required.
The availability of high-gain, high-frequency microwave transistors has revived the old "distributed" or "travelling-wave" approach for broadband microwave amplification but using GaAs FETs; see Y. Ayasli et. al., Monolithic GaAs Travelling-Wave Amplifier, 17 Electronics Letters 413 (1981). Such amplifiers are similar to the distributed amplifier using electron tubes, as described in E. L. Ginzton et. al., Distributed Amplification, 36 Proc. IRE 956 (1948), in that the intrinsic gate and drain capacitances serve as parts of the shunt elements of two artificial transmission lines: the gate and drain transmission lines. If the line element values (inductances) are chosen properly, wideband amplification can be obtained with more reasonable VSWRs than is possible for an FET having the same total gate width. The Ayasli article reported results of broadband travelling-wave amplification in the 0.5 to 14 GHz band using four discrete 300 .mu.m gate width FETs in a distributed amplifier configuration. See also U.S. Pat. No. 4,486,719 to Ayasli.